Update workspace builtin expectations and drop an unused selector

Why: Removing the artifacts builtin shifts the shortcut ranking, so the
workspaces tests now expect agents at cmd-1; Workbench no longer selects
previewOpen in its outer scope (FilesTab owns that read).

Evidence: npx vitest run 85/85 passed; npm run build (tsc -b && vite
build) passed in the task worktree (SELF).

Impact: test/workspaces.test.ts expectations; Workbench.tsx selector
cleanup. No behavior change beyond the committed layout rework.

src/components/Workbench.tsx

@@ -58,7 +58,6 @@ export function Workbench() {
   const inboxScope = useStore(s => s.inboxScope)
   const projectCards = useStore(s => s.projectCards)
   const filter = useStore(s => s.ui.inboxFilter)
-  const previewOpen = useStore(s => (s.previewTabs[previewTabsKey(s.currentConvId)]?.length ?? 0) > 0)
   const visible = inboxScope === 'project' ? projectCards : cards
   const pending = pendingCount(visible)
   // 离开文件入口时收回全屏展开，避免出现「工作」铺满主区的怪态。

test/workspaces.test.ts

@@ -1,18 +1,19 @@
 import { describe, expect, it } from 'vitest'
 import { buildWorkspaceEntries } from '../src/workspaces'
 
+// 三栏工作台（2026-10-11）：文件浏览并入第三栏「文件」入口，体系工具只补「代理」。
 describe('buildWorkspaceEntries', () => {
-  it('没有领域插件时，工件和代理补在 ⌘1 ⌘2', () => {
+  it('没有领域插件时，代理补在 ⌘1', () => {
     const entries = buildWorkspaceEntries([])
-    expect(entries.map(e => `${e.shortcut} ${e.title}`)).toEqual(['⌘1 工件', '⌘2 代理'])
+    expect(entries.map(e => `${e.shortcut} ${e.title}`)).toEqual(['⌘1 代理'])
   })
 
   it('声明了 shortcut 的工作区占住该位，内置工具让位', () => {
     const entries = buildWorkspaceEntries([
       { id: 'trace', title: '追溯网', shortcut: '1' },
     ])
-    expect(entries.map(e => e.title)).toEqual(['追溯网', '工件', '代理'])
-    expect(entries.map(e => e.shortcut)).toEqual(['⌘1', '⌘2', '⌘3'])
+    expect(entries.map(e => e.title)).toEqual(['追溯网', '代理'])
+    expect(entries.map(e => e.shortcut)).toEqual(['⌘1', '⌘2'])
     expect(entries[0].key).toBe('ws:trace')
   })
 
@@ -20,6 +21,6 @@ describe('buildWorkspaceEntries', () => {
     const entries = buildWorkspaceEntries([
       { id: 'mem', title: '记忆库' },
     ])
-    expect(entries.map(e => e.title)).toEqual(['工件', '代理', '记忆库'])
+    expect(entries.map(e => e.title)).toEqual(['代理', '记忆库'])
   })
 })